Various techniques for depositing thin films of one material on another substrate are known. Of particular attention is a technique known as pulsed laser deposition (PLD). See, e.g., Kim et al., U.S. Pat. No. 4,970,196; Wagal et al., U.S. Pat. No. 4,987,007. In PLD, a pulsed laser beam is directed at a target composed of the material to be deposited as a thin layer. Illumination of a point on the target material by the laser causes the surface of the target substrate to evaporate. The laser pulses are of short duration and high energy, so the surface boils and vaporizes before the underlying layers are heated. The emitted evaporant migrates from the target material to the surface of a deposition substrate, where it is deposited in a thin layer.
While this technique shows promise, it has not been widely used commercially, predominantly because illumination of a point or small area on the target results in uneven distribution on the deposition substrate. Upon illumination, the evaporant of the target material tends to rise from the target surface along a path substantially perpendicular to the surface of the target material, although there is some lateral spreading of the target material as it migrates. Thus the migrating evaporant takes the shape of a narrow cone (termed a "deposition plume") which is dense with evaporant toward its center (i.e., along the longitudinal axis of the cone). The density of the evaporant diminishes rapidly as a function of radial distance from the longitudinal axis of the cone and the distance from the target material. As a result, the area of the deposition substrate immediately adjacent the intersection of the deposition substrate and longitudinal axis of the plume receives considerably more target material than do areas of the deposition substrate somewhat removed from the longitudinal axis; consequently, the deposition layer formed is much thicker in its central portion than its peripheral portions. This shortcoming has restricted the commercial use of PLD to film coverage of deposition substrates with very small surface areas.
In view of the foregoing, it is an object of this invention to provide a PLD method which substantially increases the area over which a substantially constant film thickness can be deposited using PLD.
It is an additional object of this invention to provide an apparatus with which the aforementioned method can be practiced.